many other interesting proper-
ties. Higher-order topological
insulating states, where regions
of interest are along edges and
at corners, have been difficult to
identify unambiguously. Peterson
et al. developed a theoretical
framework to help identify and
characterize these exotic states,
including a new topological
marker—the fractional charge
density—that can be used to
detect topological states of
matter when the spectroscopic
probe of gapless surface states
is not accessible. The agreement
between experimental work
and theory is encouraging for
applicability to other topological
platforms. —ISO
Science, this issue p. 1114CANCER
How cancer cells adapt
to stress
Bacteria adapt to harsh
conditions such as antibioticWEARABLE DEVICES
Improving ionic
thermoelectrics
Using ions as charge carriers in
thermoelectric devices usually
requires using either thermal
diffusion or redox reactions at
two electrodes with different
temperatures. Han et al. lever-
aged both of these strategies
to develop a gelatin-based ionic
thermoelectric device that uses
alkali salts and an iron-based
redox couple to generate a large
thermopower. This device is
capable of generating useful
amounts of energy from body
heat. —BG
Science, this issue p. 1091
PLASTIC POLLUTION
Not just settling
What controls the distribution
of microplastics on the deep
seafloor? Kane et al. show that
the answer to that question is
more complicated than particles
simply settling from where they
are found on the sea surface
(see the Perspective by Mohrig).
Using data that they collected off
the coast of Corsica, the authors
show that thermohaline-driven
currents can control the distribu-
tion of microplastics by creating
hotspots of accumulation, analo-
gous to their role in causing
focused areas of seafloor sedi-
ment deposition. Such currents
also supply oxygen and nutrients
to deep-sea benthos, so deep-
sea biodiversity hotspots are also
likely to be microplastic hotspots.
—HJS
Science, this issue p. 1140;
see also p. 1055CORONAVIRUS
Smothering fecal-oral
coronavirus spread
Diarrhea is a common symptom
in patients with coronavirus
disease 2019 (COVID-19). Zang
et al. used organoid cultures ofcells from the epithelial lining of
human small and large intestine
as an in vitro model system with
which to study viral entry and
replication in enterocytes. Mature
enterocytes expressing the viral
receptor were susceptible to pro-
ductive infection, which was also
stimulated by the expression of a
protease involved in viral entry. A
subset of patients with COVID-19
shed high amounts of viral RNA
in feces, but experiments with
simulated human colonic fluid
suggested that any shed virus
would be rapidly inactivated
during transit through the colon.
—IRW
Sci. Immunol. 5 , eabc3582 (2020).TOPOLOGICAL OPTICS
Topological insulators in
the spotlight
In addition to having an insulating
interior while at the same time
supporting conducting surface
states, topological insulators have1076 5 JUNE 2020 • VOL 368 ISSUE 6495 sciencemag.org SCIENCE
RESEARCH
REPRODUCTIVE BIOLOGYLocal control of sperm maturation
N
ewly produced spermatozoa within the testis do not
have fertilizing ability but become fully functional when
they mature in the epididymis. The development of the
epididymis itself is dependent on testicular factors
arriving via luminal flow. Improper signaling between
the testis and epididymis is hypothesized to result in male
infertility. Kiyozumi et al. identified NELL2 as a testicular
luminal protein that binds to its receptor, ROS1, on the luminal
epididymis surface and induces epididymal differentiation
(see the Perspective by Lord and Oatley). In turn, differentiated
epididymis secretes a fertility-essential protease, ovochy-
mase-2, to make spermatozoa fully mature and functional.
Thus, testis-epididymis interorgan communication by this
“lumicrine” regulation ensures mammalian reproduction. —BAP
Science, this issue p. 1132; see also p. 1053CREDIT: KIYOZUMIET AL.IN SCIENCE JOURNALS
Edited by Michael FunkFluorescence microscopy image revealing localization of the protease ovochymase-2
(purple) within the caput epididimus, which is part of the male reproductive tract in mammalsPublished by AAAS